The very easiest thing to think of in this ballpark is the relative minor - the 6th degree of a major scale. Even easier than that - you can find it a tone and a half (3 frets) down from the minor key you're in...

So, if you're playing a chord progression that's E Major, you can play a C# minor scale over it...

But yeah, sus2 chords lack the all important third as dvenetian says...